As is known, ammonia is commercially produced starting from a nitrogen-hydrogen stoichiometric mixture according to the following exothermic reaction: EQU N.sub.2 +3H.sub.2 .revreaction.2NH.sub.3
Since the ammonia concentration at the equilibrium increases--according to the chemical thermodynamics laws--as the pressure increases or as the temperature decreases, it follows that if one wants to operate at lower pressures, it is advisable to employ lower temperatures to avoid an excessive productivity reduction. A progressive decrease in the synthesis pressure is being since many years a constant trend in this type of process.
More recently, the imperative necessity to reduce the energetic costs has rendered particularly convenient the development of processes characterized by synthesis pressures ranging from 50 to 100 atm., instead of the generally employed 150-300 atm. The unavoidable decrease in the reaction rate related to both the lower pressure and the lower temperature, which becomes necessary to maintain favourable equilibrium conditions, makes it difficult to practise a low pressure process (50-100 atm.) using the catalysts known at present.
Therefore it becomes necessary to find out catalysts endowed with a higher activity.
The catalysts which are known for the ammonia synthesis consist of magnetite (Fe.sub.3 O.sub.4) with the addition of small amounts (5-10% by weight) of some oxide-based promoters, difficult to be reduced, such as Al.sub.2 O.sub.3, K.sub.2 O, CaO, MgO, SiO.sub.2 and the like. The catalyst is activated in the reactor synthesis by reducing, with the nitrogen-hydrogen mixture, the magnetite to metal iron according to the endothermic reaction: EQU Fe.sub.3 O.sub.4 +4H.sub.2 .revreaction.3Fe+4H.sub.2 O.
In a few cases such reduction is effected by the catalyst manufacturer in a proper plant and the reduced catalyst is then passivated on its surface with oxygen so as to permit the handling thereof in air, as it would be otherwise pyrophoric. This preliminarly reduced catalyst is then subjected to a short-lasting reducing treatment in the synthesis reactor in order to remove the surface oxide layer.
A slight improvement in the activity of the ammonia synthesis catalyst can be obtained by addition of cobalt (up to 10% by weight). The high cost of cobalt, amounting approximately to 100 times that of iron, however, renders such a catalyst little convenient.
As is known, the catalyst preparation consists in melting the mixture in powder, which is made of magnetite and of the oxide promoters, and in successively cooling the molten mass poured into proper collecting vessels.
The resulting solid product is then crushed, ground and screened to obtain the desired sizes.
Cooling of the molten mass is generally accomplished in sheet iron pans which are lined with a few-centimeter thick magnetite layer. Such layer has an insulating effect, wherefore very low cooling rates, generally around 2.degree. C./min., are obtained.
On the other hand, in consideration of the technological problems involved by the treatment of high temperature materials, such as molten magnetite, mainly for commercial-scale preparations, the removal of the insulating layer is not possible in practice, in particular when iron cooling vessels are used, due to the low melting temperature of iron in respect of that of magnetite.
In the molten mass solidification technique it is known, from European patent application No. 82300749.7, that by pouring the melt into iron vessels having a properly grooved bottom and by selecting a suitable cooling velocity and a suitable thickness of the molten magnetite layer, it is possible to obtain, after crushing, catalyst granules having the desired size and being characterized by a roundish profile at least on one side.
The catalysts so prepared exhibit, in the ammonia synthesis, lower pressure drops and an activity exceeding about 7% the one of the present commercial catalysts.
From the foregoing it is apparent that the cooling velocities--not exactly indicated--attained according to the abovesaid technique employing iron vessels can be only of a few centrigrade degrees per minute.
In Bull. Chem. Soc. Japan, Vol. 29, pages 20-27, 1956, it is indicated that the quick cooling, by means of quenching techniques, of the molten catalyst does not lead to any improvement in the catalytic activity in the ammonia synthesis with respect to the usual cooling. It is furthermore indicated that a long-lasting catalyst annealing during 40 hours at 1000.degree. C. causes a reduction in the catalyst activity.